1. Field of the Invention
The present invention relates to a rib of a composite material for a box structure, such as an airfoil, and a method of forming such a rib by molding.
2. Description of the Related Art
A known technical measure for enhancing the buckling strength of a channel-shaped rib for an aircraft structure or the like forms beads, i.e., strengthening protrusions, perpendicularly to the length of the rib.
As shown in FIG. 12, a channel-shaped rib 1 has a web 2, flanges 3 formed so as to extend along the opposite longitudinal side edges of the web 2, and a plurality of ridges or beads 4 formed in the web 2 at intervals. If the beads 4 are formed when forming the rib 1, the arc length L1 (FIG. 13) of the beads 4, i.e., length across the bead along the surface of the web 2, is greater than the chord length L2 of the same along the surface of the flange 3, i.e., width.
When forming the rib 1 shown in FIG. 12 by molding a prepreg plate 5 (FIG. 14) prepared by impregnating carbon fiber fabrics or glass fiber fabrics with a synthetic resin by a molding process, the prepreg plate 5 is unable to absorb the difference between the arc length L.sub.1 and the chord length L2 because the prepreg plate 5 is scarcely stretchable when the beads 4 are formed. Consequently, wrinkles are formed in portions of the web 2 around the corners of the beads 4 of the rib 1 and it is very difficult to smooth the wrinkles in the molding process. A technical measure for preventing the formation of wrinkles in the portions of the prepreg plate 5 around the corners of the beads 4 forms cuts 6 in the portions of the prepreg plate 5 around the corners of the beads 4 as shown in FIG. 14 to absorb the difference between the arc length L.sub.1 and the chord length L.sub.2 when forming the beads 4.
JP 3-83624A and 3-126532A disclose composite material molding methods which form strengthening ridges in the web by disposing a prepreg plate with its fibers extended bias, i.e., substantially at an angle of 45.degree., to the edges along which the web and the flanges intersect and by molding the prepreg plate, using a mold and an auxiliary mold.
If the channel-shaped rib 1 shown in FIG. 12 is to be used as a rib for a box structure, such as an airfoil, in most cases, a recess 7 (FIG. 15) is formed in the web 2, the rib 1 is attached to a skin plate 8, and a longitudinal member 9 is attached to the skin plate 8 so as to extend through the recess 7 formed in the web 2.
The technical measure which forms the cuts in portions of the prepreg plate around the corners of the beads before subjecting the prepreg plate to a molding process needs an additional cutting process, reduces the strength of the portions of the prepreg plate around the corners of the beads because the component fibers in the same portions of the prepreg plate are cut, increases the number of the laminated component prepreg sheets of the prepreg plate to compensate for the reduction of the strength of the portions of the prepreg plate around the corners of the beads due to the formation of the cuts, inevitably needs increased time and labor for fabricating the rib of the composite material, and increases the weight of the rib.
The method of forming a rib of a composite material which forms reinforcing ridges, i.e., beads, in the web of the rib enables smoothing out wrinkles if the prepreg plate is disposed with the reinforcing fibers thereof extended bias, i.e., substantially at an angle of 45.degree., to the edges along which the web and the flanges intersect. However, if the prepreg plate is disposed with its reinforcing fibers extended in parallel or perpendicularly to the edges, large wrinkles are formed because wrinkles cannot be smoothed out and the difference between the arc length of the beads along their surfaces and the chord length, i.e., the width, of the beads along the flange cannot be absorbed. Furthermore, if the recess 7 is formed in the web 2 to use the rib 1 as a member of a box structure, such as an airfoil , the strength of the web 2 is reduced significantly and cracks may develop.